1. Field of Technology
This technology relates to the polarization of light with less attenuation than normally associated with absorptive or reflective polarizers.
2. Description of the Related Art
The vast majority of liquid crystal devices in use around the world use absorptive polarizers, which attenuate slightly more than 50% of the light passing through them by absorption. Typically these are Polaroid films made, for example, from iodine-impregnated polymers stretched along one axis.
Wire-grid polarizers (WGPs) that reflect infrared light, rather than absorbing it, have been described since the 1960s, for example, in U.S. Pat. No. 4,512,638 to Sriram, et al. Such a device consists of a sub-wavelength scale array of closely spaced, parallel metal wires on a transparent substrate, such that light of one linear polarity that strikes the wires is reflected while light of opposite linear polarity is transmitted through the substrate. With the advent of nanoscale lithography in the 1990s and 2000s it became possible to produce broadband wire-grid polarizers that can polarize and reflect all the way up into visible and ultraviolet wavelengths for use with high-end optics, projective LCD video displays, and laser technology, as described for example in U.S. Pat. Nos. 6,122,103 and 6,288,840 to Perkins, et al.
More recently, low-cost reflective polarizer films combining the properties of a layered-polymer distributed Bragg reflector (DBR) with a stretched-polymer polarizer have been introduced. Such reflective polarizers are used in video displays to enhance brightness by recapturing, rather than absorbing, the attenuated light, as described for example in U.S. Pat. No. 7,038,745 to Weber, et al. and U.S. Pat. No. 6,099,758 to Verrall, et al. Such reflective polarizers can exhibit specular reflection, as in a mirror, or diffuse reflection, as in a coating of white paint, or a combination of the two.
In addition, reflective polarizers can be made from certain types of liquid crystals. Whereas wire-grid polarizers and stretched polymer polarizers are linearly polarizing, these liquid crystal polarizers (LCPs) are generally circularly polarizing. Thus, light of one circular polarization (i.e., right-handed or left-handed) is transmitted and light of the opposite circular polarization is absorbed or reflected.
Reflective polarizers of various types are a component of liquid-crystal-based video displays and thermoreflective optical filters. Typically these are linear rather than circular polarizers, as high contrast ratio and broad viewing angles may be more difficult to achieve using circular polarizers.
In addition, there are numerous examples of polarity-rotating materials, also known as “retarders” or “waveblocks” or “waveplates”. In some cases these are structured devices such as twisted nematic liquid crystal cells or liquid crystal polymers, but more frequently they are simply birefringent materials, i.e., materials which exhibit a direction-dependent index of refraction. Such devices typically act across a range of wavelengths and, within that range, they operate equally on all polarities of light and act reversibly such that a photon passing through in one direction may be rotated clockwise whereas a photon passing through in the opposite direction may be rotated counterclockwise. In contrast, a “Faraday rotator” rotates the polarization of light in a non-reversible way. In other words, a photon passing through the Faraday rotator in one direction and then reflected back in the other direction experiences double rotation rather than net-zero rotation. However, even very efficient Faraday rotator materials such as terbium gallium garnet require strong magnetic fields and long optical paths in order to achieve meaningful rotation, making them impractical for most applications.
There are other examples of “metamaterials” or nanostructured materials incorporating sub-wavelength features that interfere with light waves in such a way that the metamaterial has an “effective permittivity” ∈eff, “effective permeability” μeff, and “effective index of refraction” neff, and thus a “wave impedance”
      Z    =                            ɛ          eff                          μ          eff                      ,that are quite distinct from those of the substances from which the metamaterial is made. Depending on the structure of the device (particularly features with inductive and capacitive properties), these parameters can even be simultaneously negative—something that does not occur in natural materials. Thus, using metamaterials it is possible to construct devices that “violate” the “laws” of classical optics, including achieving resolutions significantly higher than classical diffraction limits and extending near-field features into the far field. However, metamaterials are generally transmissive only when ∈eff, and μeff have the same sign, i.e., “double positive” (alternatively “right-handed” or “positive index”) and “double negative” (alternatively “left-handed” or “negative index”) materials are transmissive to some degree, whereas “single positive” materials are opaque. One example of a device with such a metamaterial is a planar microwave antenna with negative ∈ and μ based on a meander line or space-filling curve. See, Barbagallo, S., et al., “Synthesis of novel metamaterials,” Chapter 2 (VDM Verlag 2008).
Other exemplary metamaterials are based on a transmission line topography periodically loaded with series capacitors and shunt inductors. See, Iyer, “Negative refraction metamaterials,” Chapter 1, (Wiley 2005). This structure makes it possible to control ∈eff, μeff, and neff for positive, negative, or mixed values by adjusting the values of the capacitance and inductance of each periodic unit cell within the transmission line, and to adjust the wavelength range over which the device operates by adjusting the size of the unit cells. A “plasmonic nanowire composite metamaterials” is described that consistsdx of metallic nanowire segments distributed on or within a dielectric substrate, either randomly or periodically and either singly or in pairs See, “Negative refraction metamaterials,” Chapter 8, Sarychev et al. (Wiley 2005). Depending on the length, diameter, concentration, and spacing of the wire segments, the ∈eff, μeff, and neff of the composite material can, for a given range of wavelengths, be adjusted to positive, negative, or mixed values.
Various planar, diagonally-symmetric “unit cells” or “particles” or “artificial atoms” that consist of planar arrangements of metal wire on a transparent dielectric substrate which, when arranged in regular 2-dimensional arrays, yield metamaterials with various properties including negative permittivity over certain ranges of wavelengths. See, Padilla, W. J., et al., “Electrically resonant terahertz metamaterials: Theoretical and experimental investigations” Physical Review B 75, 041102(R) (2007). These properties are dependent primarily on the size and spacing of the unit cells and secondarily on the shape of the unit cells. In general, the sizes of these unit cells fall between one-sixth and one-twentieth of the wavelengths they are intended to operate on.
Mismatched values of ∈eff and μeff, (i.e., where one parameter is close to that of free space while the other has a large positive or negative value) can rotate the polarity of a photon by shifting its electric and magnetic phases by different amounts. Thus, a metamaterial of this type can serve as a kind of waveblock, i.e., a device that rotates the polarity of photons within a particular wavelength range across a particular distance by a particular amount. These effects are frequency dependent, and various frequency responses can be arranged through design.
In addition, metamaterials can be designed for which the effective permittivity, permeability, and refractive index (and thus the overall optical properties) are different depending on the polarity of the incident light. One example of such a design is a chiral, planar structure consisting of “fish scale” patterns of aluminum nanowire on a transparent substrate. See, Fedotov, V. A., et al., “Asymmetric propagation of electromagnetic waves through a planar chiral structure,” Physical Review Letters 97, 167401, (17 Oct. 2006). In essence, the structure is a type of wire grid polarizer, although it reflects and transmits circularly polarized light rather than linearly polarized light. Because its chiral nature is different depending on which surface of the polarizer is being observed, for light of mixed, random polarity (e.g., sunlight), this structure has an additional property of being asymmetrically transmissive, i.e., it is more transmissive to light passing through it in one direction than in the other.
Another example is a bi-layered metamaterial that is capable of altering (rotating) the azimuth of circularly polarized light in a manner comparable to a retarder or waveplate although in a much thinner structure. See, Rogacheva, A. V., et al., “Giant gyrotropy due to electromagnetic-field coupling in a bilayered chiral sStructure,” Physical Review Letters 97, 177401 (27 Oct. 2006). A further example is a planar, chiral metamaterial that is capable of rotating the azimuth of circularly polarized light, in such a way that the degree of rotation depends on the input azimuth. See, Zhang, W., “Giant optical activity in dielectric planar metamaterials with two-dimensional chirality,” Journal of Optics A: Pure and Applied optics, 8, pp. 878-90 (2006). Practical applications are not discussed for either device. Both are chiral and therefore non-axisymmetric, which limits their manufacturability. In addition, because they are chiral these structures act on circularly rather than linearly polarized light, which limits their potential utility in devices such as video displays and optical limiters for the reasons described above, i.e., because high contrast ratio and broad viewing angle are more difficult to achieve using circularly polarized light.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the invention is to be bound.